This invention relates to the quantitative determination of chemical oxidizing and reducing agents in a fluid (gas or liquid) environment. More particularly, the invention relates to the determination of chemical oxidizing and reducing agents by means of a sensor or detector apparatus employing an ion-exchange membrane.
A sensor apparatus of this type is disclosed in commonly-assigned U.S. Pat. No. 4,333,810 to Wolcott et al. (the '810 patent), wherein a tubular ion-exchange membrane is employed to contain an electrolyte solution, and to separate a first electrode in contact with its fluid environment and wrapped around the membrane and a second electrode positioned in the electrolyte solution. Means are provided for measuring a flow of electrical current between the first and second electrodes attributable to the oxidizing or reducing agents coming into contact with the first electrode, for example, a microammeter or resistor in parallel combination with a voltmeter. An additional means for imposing a voltage across the electrodes is provided in certain embodiments, e.g., a battery or an alternating power source stepped down with a direct current transformer or rectifier, and a chart recorder or the like may be employed in conjunction with the current measuring means.
A second membrane-based, galvanic-type sensor apparatus known to us employs a flat membrane for containing an electrolyte solution and for separating a first electrode in contact with the fluid environment and a second electrode in the electrolyte solution. The first electrode in this flat-membrane design is in the form of a flat wire mesh, and the remainder of the apparatus apart from the sensor proper may be as described in the '810 patent.
An amperometric variation of this second, flat-membrane design employs a third, driven electrode to supply the current that would otherwise be supplied by the corrosion of the second or reference electrode, whereby the effective lifetime of the second electrode may be substantially extended. This three-electrode, flat-membrane sensor apparatus is depicted in FIG. 1, and its construction and manner of operation will be described in detail below.
As discussed in the '810 patent, the art prior to the '810 patent had the electrodes forming a part of the electrochemical cell in the sensors separated from each other by a porous layer. This porous layer design, however, permitted a substantial diffusion of a sample throughout the electrolyte between the electrodes, so that particularly after exposure to a high concentration of the particular oxidizing or reducing agent in question a long recovery time was required to stabilize the sensor and to again enable the detection of lower concentrations. The porous layer was also non-selective, and allowed interfering or poisonous species to pass freely into contact with the electrodes.
The sensor apparatus in the '810 patent and the two- and three-wire flat membrane sensor apparatus described above don't have the lengthy recovery and contamination problems associated with the previous electrochemical sensors, but have not overcome another significant problem with the known sensor apparatus of oxidizing and reducing agents such as chlorine. Typically these sensor apparatus detect chlorine through the reduction half-reaction of chlorine, and water participates in this reduction.
As a consequence of this participation, all of the previously known chlorine sensors have a degree of sensitivity to fluctuations in the water content of the sensor's immediate environment. One significant use of chlorine sensors is as perimeter monitors for atmospheric chlorine. In those climates which are characterized by cold, dry winter climates for example, the known chlorine sensors have largely been rendered ineffective. For the same reasons, chlorine sensors have heretofore not proven useful as process monitors in the monitoring of anhydrous or low water-content process streams.